An internal combustion engine has been widely used in a vehicle, a ship, a small power generator, and the like, and thus, an attempt to improve efficiency of the internal combustion engine has been continuously conducted. In the internal combustion engine, a large quantity of heat is exhausted as waste heat, and a waste heat recovery unit (WHRU) for increasing entire efficiency of the internal combustion engine by recovering the waste heat has been developed.
The WHRU recovers energy from the waste heat exhausted from the engine, converts the recovered energy into electric energy or mechanical energy, and utilizes the electric energy or the mechanical energy in the engine, other electrical accessories, or the like, of the vehicle.
The WHRU uses a Rankine cycle system to effectively recover the waste heat of the engine. The Rankine cycle system for waste heat recovery includes a circulation path through which an operating medium is circulated, and the circulation path of the Rankine cycle system includes an evaporator (boiler) heating and evaporating the operating medium by the waste heat (heat of an exhaust gas and/or heat of an exhaust gas recirculation (EGR) gas) of the engine. An expander expands the operating medium supplied from the evaporator to generate rotation power. A condenser condenses the operating medium exhausted from the expander, and a pump circulates the operating medium on the circulation path.
In the WHRU according to the related art, the expander of the Rankine cycle system is directly connected to an output shaft of the engine through a drive mechanism, such as a gear driver, a belt driver, or the like. Thus, the recovered power of the expander is supplied as an assist power to the engine.
When the expander of the Rankine cycle system is directly connected to the output shaft of the engine through the gear driver, the belt driver, or the like, as described above, the recovered power of the expander is lost during deceleration or braking of the engine.
In addition, when the recovered power of the expander is not sufficiently recovered, such that a revolutions per minute (RPM) of the expander is not appropriately controlled depending on a change in an RPM of the engine, noise and vibrations are generated.
Particularly, when the RPM of the expander is slower than that of the engine, the recovered power of the expander does not assist rotation of the engine, but the engine rotates the expander to decrease fuel efficiency of the engine, which is a major problem in the engine.